Well slim that all depends on what you mean by small. Small in image size does not necessarily mean small filesize. Also you should always resize by pixels not inches, because realworld inches don't always translate to screen inches.
Also file type is a big determining factor. For a nice small (file size) image go with a JPG.
If you want to resize the image in proporations i'd go with something less than 1024x768 pixels for posting images to boards. 1024x768 is a pretty standard resolution for monitors, so that would roughly fill alot of peoples screens. A 1024x768 JPG would be about 200k.
When you save a JPG in photoshop you get an option to choose quality, the lower the quality the smaller the filesize, but don't make it too low or you will be able to see blocky artifacts on the image, anywhere 8 and up is pretty much undetectable on most screens.
And finally the step by step part:
Open the image in Photoshop.
Go to the Image menu, select Image Size.
When the new window opens, make sure Pixels is selected in the dropdown menu, and make sure the "Constrain Proportions" Check box is checked.
Type in for width whatever you want the width to be (less than 1024 recommended), The height should automatically type in a correct value.
Hit OK.
Go to file, save as.
Select Jpg from the dropdown menu, and type in a file name, Hit save.
A new window should pop up after a few seconds. Select the quality in the slider ( 8 or higher recommended), Hit OK.
Enjoy! Any questions just ask
